Introduction

Fatty acids are a kind of fat or lipid. The term “lipid” is rather broad and can refer to a variety of compounds that share similar properties of hydrophobicity (insoluble in water). Fats are generally soluble in organic solvents. Chemically fats are tri-esters of glycerol and fatty acids. Fats (lipids) are important molecules that serve both structural and metabolic functions of animal systems (Leeson and Summers, 2001). Lipids play important roles both physically and physiologically when added in the diets of poultry birds. From a physical aspect, fats help in the improvement of feed quality through dust reduction in feed, decreasing feed particle separation, and increasing palatability as well as digestibility of feed (Leeson and Summers, 2001). The physiological role of lipids includes energy production, nutrient transport, and cellular structure (Leeson and Summers, 2001). Nutritionally, fats increase the caloric density of feeds, providing essential fatty acids and fat-soluble vitamins and pigments.

The importance of fat led to the establishment of essential fatty acid requirements for poultry birds. Subsequent studies were conducted to establish a recommended level of linoleic acid for growing poultry. In this regard poly unsaturated fatty acids (PUFA), divided into omega 3 (n-3) and omega 6 (n-6), gained much attention. The PUFAs affect physiological and pathophysiological processes leading to effects in health. Metabolic function, reduction of the serum cholesterol concentration, neuronal development, prevention of cardiovascular disorders, improvement of immune response, and insulin action have been closely related to n-3 PUFAs (Phillipson et al., 1985; Leaf and Weber, 1988; Simopoulous, 1991; Leaf and Kang, 1998, Stulnig, 2003).

The importance of fatty acids, particularly PUFA, has increased the concern of researchers to study the effects of dietary fat supplementation on performance, fatty acid (FA) composition of tissues, products and immune system of poultry birds. Consequently, there has been a great interest in using different types of oil seeds as a source of varying types of fats and oils as additives in poultry diets. Likewise, enrichment of poultry products (meat and eggs) with PUFA through addition of fats or oils rich in PUFA has become a routine practice. These enriched products gained popularity as functional food for human consumption. During the course of production of fatty acid enriched poultry products, the feeding of different types of fat and oils exert both beneficial and detrimental effects on the growth, productivity and health status of poultry birds. This review will discuss the role of fatty acids feeding on the live performance, meat and egg production, and immune function of poultry birds.

 Fatty acids and their sources

Fatty acids may be of different chain lengths, degrees of saturation and configurations. Fatty acids, on the basis of absence or presence of double bonds, are classified as saturated or unsaturated. Fatty acids which possess no double bonds are grouped as saturated, for example palmitic acid. Fatty acid such as oleic acid contain only one double bond are classified as monounsaturated FA (MUFA). The PUFAs, such as linoleic acid (LA) eicosapentaenoic acid (EPA) and arachidonic acid (AA), contain two or more double bonds (Heird and Lapillonne, 2005). According to the position of the first double bond relative to the terminal methyl end of the molecule, PUFAs are divided into omega 3 (ω-3/n-3) and omega 6 (ω-6/n-6) FA. The trivial and systemic name, short designation and sources of fatty acids are given in Table-1.

The saturated FAs and MUFAs are de novo synthesized by all mammals from simple precursors such as glucose or ketogenic amino acids. However, they are not able to endogenously synthesize either (LA, 18:2n-6) nor α-linolenic acid (ALA, 18:3n-3); thus they are considered essential fatty acids (EFAs). Likewise, avian species are also unable to synthesize ALA  and  LA, thus have to be supplied in the diet (Cherian, 2011). Although ALA and  LA cannot be synthesised by the bird, however upon consumption, they can be elongated into a variety of 20 and 22-carbon FAs.

Commercial poultry diets vary in composition, but they possess the basic nutrients to provide chicks with the appropriate level of nutrients required for growth. Corn, safflower, soybean, and sunflower oils are rich sources of LA. Oils from flax (Linum usitatissimum), chia (Salvia hispanica) and canola (Brassica napus) are rich in ALA (Shaikh and Edidin, 2006, Cherian, 2011,).  Among the different plant based sources, flaxseed, owing to its metabolisable energy value (>2000 kcal/kg), protein (>22%), fat (>38%) ALA (>50%), and availability, is the most common ingredient used as n-3 fatty acid source (Cherian, 2008).

 Fatty acids and growth

Fat or oil is commonly added in poultry diets as a source of concentrated energy and EFAs. Dietary fat inclusion can be reduced to a low level but it cannot be entirely eliminated as poultry birds are unable to synthesize EFAs. Owing to high growth rate, broiler chickens have a high requirement for energy and the use of fat (triglycerides) as a major energy source might result in a fat carcass.  However, Newman (2000) reported that feeding of n-3 (fish oil) and n-6 (sunflower oil) fatty acids resulted in a leaner bird having non-significant difference in weight gain with an accompanying improvement in feed conversion efficiency. This indicated changes in avian metabolism by n-3 and n-6 fatty acids through the modulation of lipid deposition and oxidation (Newman et al., 2002).

Coetzee and Hoffman (2002) used different combinations of canola acid oil (high levels of C18:3n-3 and MUFA) and famarol acid oil (high levels of C18:2n-6 and saturated FAs) in broiler diets and noted non- significant differences between dietary groups in weight gain or feed conversion ratios. Febel et al., (2008) used lard, sunflower oil, soybean oil and linseed oil in broiler diets and noted non-significant effect on growth performance and feed intake. Pinchasov and Nir (1992) also showed that dietary fat has no effect upon performance in broilers when the energy to protein ratio and the other of nutrients were maintained. Crespo and Esteve-Garcia (2002a) used 10% of added tallow, soybean oil and linseed oil and Sanz et al. (1999) used 8% of tallow, lard and sunflower oil did not noticed any differences in the productive performance of broiler chickens. Kavouridou et al., (2008) fed female broiler chickens with different fats sources viz., coconut, palm, olive, soybean and linseed oil, at 10% level and noted non-significant differences on productive performance. Similarly, non-significant change in weight gain was reported by Viveros et al., (2008) and Al-Khalifa et al.,  (2012) who fed different fat sources (higholeic-acid sunflower seeds,  palm oil, high-oleic-acid sunflower oil and high-oleicacid sunflower hulls) and fish oil (up to 60g/kg of diet), respectively. However, Pinchasov and Nir (1992) and Zollitsch et al. (1997) showed an improved gain to feed ratio in broilers when dietary PUFA intake increased, which could be attributed to greater intestinal uptake of unsaturated fatty acids. Similarly, Newman et al. (2002) found a better food conversion ratio in broilers fed sunflower oil compared to those fed tallow, even though statistical differences in food intake and weight gain were not detected

It has been noted in different studies that body fat contents changed with the source of fatty acid. The intake of PUFA compared to the intake of saturated fatty acids caused a lower fat deposition in the animal (Villaverde et al., 2005). Kavouridou et al., 2008 body fat content of the animals consuming the more linseed oil (more unsaturated fat) was lower than the body fat of the animals fed the coconut, palm and olive oil (MUFA and saturated FA).

In general, feeding broilers with different fat sources resulted in non-significant effect on growth rate. However, some improvement in feed to gain ratio was noted in few studies. Birds kept on more PUFA diets showed less body fat that resulted in learner carcasses than those on saturated fat.